Ours is the Fury doesn't care about challenge icons or stealth. Those requirements are only checked during the "normal way" to add characters to a challenge, during the"declare attackers" and "declare defenders" frameworks, whereas this ability is an action performed during an action window within a challenge.

For example, Robert Baratheon can be made to participate in an intrigue challenge with Ours is the Fury's ability.

Likewise, if Robert is stealthed, he can still be added as a defender with Ours is the Fury (provided he was already kneeling).

If Robert is already kneeling as a defender, can he be targeted by this card? Although the sentence "that character is now participating as a defender" will not change the game state, the last sentence (about standing) still has the possibility to change the game state.
—
Prof*Joe11/31/17

Yes, you can use Ours is the Fury on an already-defending character,
—
scantrell2427191/31/17

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